1. Field of the Invention
This invention relates generally to a video signal processing circuit and, more particularly, is directed to a video signal processing circuit for use in a television receiver.
2. Description of the Prior Art
When a video signal is applied to drive the cathode of a cathode ray tube, the level thereof is selected so that the picture screen of the cathode ray tube becomes darkest at the pedestal level V.sub.1 of the video signal and brightest at the white level V.sub.2 of the video signal, as shown in the graph of FIG. 1. This is accomplished by setting the cut-off level (black level) of the cathode ray tube equal to the pedestal level of the video signal, thus restoring the DC component thereof. In this situation, any change from station to station of the set-up level of the video signal, that is, the small difference between the pedestal level and the black signal level, is manifested as a change in the black signal level. The scattering or variation of set-up levels between stations (channels) appears as a scattering or variation of the black level. Therefore, when a broadcast signal of a certain station having a high set-up level is received, the black parts in the reproduced picture appear somewhat greyish. For this reason, in a practical embodiment of a television receiver, the DC transmission factor thereof is lowered to a value in the range of about 0.5 to 0.9 to reduce fluctuation of the black level caused by the scattering of the set-up level. However, the fluctuation of the black level cannot sufficiently be removed in this manner.
In order to solve the above problem, it has been proposed to detect the level of the video signal which is nearest to the black side and which is nearer to the black side than a predetermined level in a video period, to define this detected level as the black level and to set this detected level as the cut-off level of the cathode ray tube.
In other words, in such proposed system the level of the video signal nearest to the black side is defined as the black level such in a video period that the level of the video signal in one horizontal period which is nearest to the black side, as shown by the broken line in FIG. 2A, is detected, and the detected level, is lowered to the pedestal level as shown in FIG. 2B. However, according to the above signal processing system, a video signal having an APL (average picture level) which is relatively high, as shown in FIG. 3A, and having small level fluctuation, is converted into the video signal shown in FIG. 3B, which has an entirely different luminance or brightness level than that of the original video signal. Therefore, it has been proposed to define a predetermined level (for example, corresponding to 20% of the APL) of the video signal, as indicated by the one-dot chain line in FIG. 3A, as a threshold level Vth, and the level of the video signal at the side more black than the threshold level Vth is detected and only the detected signal portion is processed.
Even if the signal is processed according to the above proposal, problems may still occur. One problem occurs in the case where the set-up level is abnormally high, as shown by the broken line in FIG. 3C, that is, where the blackest level of the video signal is not low in comparison with the threshold level Vth. In such case, the reproduced picture generally becomes white, and accordingly, has a low contrast. In this case the reproduced picture is improved by lowering the level of the signal nearest to the black side to eliminate the set-up level. Another problem occurs in the case where the APL of the video signal is dark grey with approximately less than 10% variation and a pulsating signal of high level is included in the video signal, as shown in FIG. 3D. In this case, if the signal portion with a level lower than the threshold level Vth is lowered to the pedestal level, almost all of the reproduced picture becomes black with the result that bright signal information is reduced. This may be mistaken as a malfunctioning television receiver.